1. Field of the Invention
The present invention relates to a driving circuit, and more particularly to a current driving circuit most suitable for driving a printing head of a printer.
2. Description of the Prior Art
FIG. 1 shows a prior current driving circuit for heating, for example, a heating element of a printer thermal head.
The current driving circuit has a complementary MOS(CMOS) inverter 10 composed of P and N type MOS FETs 12 and 14, a Darlington circuit 20 including NPN type bipolar transistors 22 and 24, a current limiting resistor R.sub.1, and pull-down resistors R.sub.2, R.sub.3.
An output terminal 18 of the current driving circuit is connected with a resistance element R.sub.l which, for example, corresponds to one of the heating elements of a thermal head.
In operation, when an "H" level input signal V.sub.IN is inputted to an input terminal 16, the CMOS inverter 10 outputs an "L" level signal, for thereby allowing a transistor 22 in the Darlington amplifier 20 to be switched off. Accordingly, no current flows through the resistance element R.sub.l and thus results in no heat production.
Then, when an "L" level input signal V.sub.IN is inputted to the input terminal 16, the CMOS inverter 10 outputs a "H" level signal, for thereby allowing the transistor 22 in the Darlington amplifier 20 to be switched on. Accordingly, a current flows through the resistance element R.sub.l, for thereby causing heat to be produced to permit a picture element dot to be printed on a heat-sensitive paper sheet on a thermal printer.
However, in the prior art current driving circuit, when the resistance element R is heated, the output current of the CMOS inverter 10 flows through the P type MOS FET 12 and the resistors R.sub.1 and R.sub.2 from a power source +V.sub.dd to the ground potential, while the emitter current of the NPN type bipolar transistor 22 flows through the resistors R.sub.1 and R.sub.3 to the ground potential. Therefore, the prior art current driving circuit has a disadvantage in that power consumption is large in a printing mode of the printer.
For example, a thermal head which prints any information on a B4 type (256 mm wide) heat-sensitive paper sheet at a printing speed of 8 dots/mm includes 2048 heating elements therein, and a reactive current of approximately 430 mA flows therethrough in a printing mode if it is assumed that R.sub.2 =R.sub.3 =10 K.OMEGA..
Though the reactive current can be reduced with increased resistance values of the pull-down resistors R2 and R3, the operational speed is lowered and the noise margin would be narrowed due to degraded B.sub.CER voltage withstanding characteristics of the NPN transistors 22 and 24.